Method for producing borosilicate glasses

ABSTRACT

The process of producing a refined borosilicate glass includes preparing a glass batch with a composition in wt. % on the basis of oxide content of SiO 2,  65-82; Al 2 O 3 , 2-8; B 2 O 3 , 5-13; MgO+CaO+SrO +BaO+ZnO, 0-7; ZrO 2 , 0-2; and Li 2 O+Na 2 O+K 2 O, 3-10; adding 0.05 wt. % to 0.6 wt. % of sulfate(s) expressed as SO 3  to the glass batch as the refining agent; melting the glass batch including the refining agent to form melted glass; and then hot-shaping the borosilicate glass. The refining agent may also include from 0.01 wt. % to 0.6 wt. % of F −  or from 0.015 wt. % to 0.6 wt. of Cl − . The sulfate is preferably an alkali metal and/or alkaline earth metal sulfate or sulfates.

CROSS-REFERENCE

This is the U.S. National Stage of PCT/EP 02/06235 filed Jun. 7, 2002,which, in turn, is based on European Patent Application EP 01 114 173.6,filed on Jun. 12, 2001, in Europe.

BACKROUND OF THE INVENTION

The invention relates to a process for producing borosilicate glassesusing a refining agent for batch preparation. The invention relates to aprocess for producing borosilicate glasses with a high chemicalresistance, especially with a hydrolytic stability belonging tohydrolytic class 1.

Processes for producing glasses comprise the process steps of batchpreparation, batch charging into the melting end, melting of the glassand subsequent hot-shaping of the glass. In this context, the termmelting also encompasses the steps of refining, homogenization andconditioning for further processing which follow the operation ofactually melting down the batch.

The term refining, with regard to melts, is understood as meaning theremoval of gas bubbles from the melt. Thorough mixing and degassing ofthe molten batch is required in order to achieve the maximum freedomfrom foreign gases and bubbles. The behavior of gases and bubbles inglass melts and also the way in which they are removed are described,for example, in “Glastechnische Fabrikationsfehler”, edited by H.Jebsen-Marwedel and R. Brückner, 3^(rd) edition, 1980, Springer-Verlag,pp. 195 ff.

The chemical refining processes are the most frequently used refiningprocesses. The principle of chemical refining processes is for the meltor even the batch to have added to it

-   -   compounds which break down in the melt and thereby release        gases, or    -   compounds which are volatile at relatively high temperatures, or    -   compounds which release gases in an equilibrium reaction at        relatively high temperatures.

This increases the volume of bubbles which are present and forces themupward. The latter group of compounds includes what are known as theredox refining agents, such as for example antimony oxide, arsenicoxide. In these processes, which are by far the most common in practice,the redox refining agents used are polyvalent ions which can occur in atleast two oxidation states which are in a temperature-dependentequilibrium with one another, with a gas, generally oxygen, beingreleased at high temperatures.

The second group of compounds, namely those which are volatile at hightemperatures on account of their vapor pressure, causing their action tobe implemented, includes, for example, sodium chloride and variousfluorides. They are collectively known as evaporation refining agents.

Redox and evaporation refining are linked to the temperatures at whichthe corresponding redox or evaporation (or sublimation) processes takeplace on account of the thermodynamic conditions. For many glass melts,such as the melts of soda-lime glasses and other relatively low-meltingglasses (e.g. borate glasses, lead glasses), these options aresufficient.

However, the bubbles are more difficult to remove from glasses withmelting temperatures (temperature at which the viscosity is approx. 10²dpas) of between approx. 1550° C. and 1700° C., which means refiningtemperatures of more than 1600° C. if sufficient refining is to beachieved, on account of the increased viscosity of the glass melt. Thebubbles have less tendency to grow and do not rise upward to the sameextent as when the viscosities are lower. This leads to the formation offine bubbles which can only be removed with difficulty or can no longerbe removed at all by reducing the throughput or by means of highertemperatures, which makes these glasses unusable, since the resorptioneffect of the chemical redox refining agents, e.g. Sb₂O₃, i.e. theability to reabsorb the oxygen or other gases from the fine bubblesduring cooling and thereby to remove these gases, is insufficient formany high-melting glasses.

Moreover, the possibilities of increasing the temperatures for thepurpose of reducing viscosity and of extending the melting and refiningtimes, which in principle exist to a certain extent, are uneconomic.Excessively high melting temperatures would cause the refractorymaterial of the tank furnace to be excessively attacked, leading toglass defects and to a shortening of the tank furnace service life. Ifthe melting and refining times were to be lengthened, the meltingcapacity would be too low.

A further drawback of many redox refining agents and evaporationrefining agents is that they are environmentally harmful or at least notenvironmentally friendly.

This applies, for example, to arsenic oxide and also antimony oxide.Alternative redox refining agents, for example cerium oxide, arerelatively expensive replacement substances.

The abovementioned high-melting glasses with melting points of approx.1600° C. also include the borosilicate glasses. On account of their lowlevel of interaction with the environment, what are known as neutralglasses from the group consisting of borosilicate glasses, i.e. glasseswith a high hydrolytic stability, specifically belonging to hydrolyticclass 1 (DIN ISO 179), are of particular importance for manyapplications.

The first type of chemical refining, i.e. refining by means of compoundswhich decompose and thereby release gases, includes sulfate refining.This method too is known for low-melting glasses, for example forsoda-lime glasses used for bottle or window glass, for example, sincethe Na₂SO₄ which is customarily used (in the case of mass-producedglasses also in the form of Glauber's salt Na₂SO₄·10 H₂O) reacts withthe SiO₂ which is always present even at low temperatures, compared withNa₂SO₄, which on its own is relatively stable, in accordance with thefollowing reaction schemeNa₂SO₄+SiO₂→Na₂O·SiO₂+SO₃.

SO₃ reacts further to form SO₂ and ½ O₂, which represent the actualrefining reagents.

The purpose of the refining agent during the chemical refining processusing sulfate is to remove the gases which are released during themelting process. The refining gases have to be homogeneously physicallydissolved in the glass melt, known as the rough melt, at relatively lowtemperatures, so that it can then release the gas as bubbles at highertemperatures. The formation of gas bubbles by the refining agent ishighly temperature-dependent; the temperature influences not only theviscosity of the glass melt but also the physical solubility of thegases in the glass. If the temperature rises in the refining phase, thesolubility of the gases in the glass drops and bubbles are formed onaccount of the supersaturation at elevated temperature. The gasesreleased in bubble form from the refining agent increase the size of thesmall gas bubbles which have remained from the melting process andthereby allow them to rise up so that they can be removed from the melt.However, this requires sufficient refining gas to be dissolved in theglass to then be released at the higher temperature, the refiningtemperature.

The solubility, i.e. in this case the SO₂ solubility, however, isdependent not only on the temperature but also on the basicity of theglass.

The soda-lime glasses which it is known can be successfully refined withsulfate are glasses with a high alkali metal content and a highalkaline-earth metal content. These glasses are basic on account of thehigh alkali metal content. They therefore have a high SO₂ solubility,likewise on account of the high alkali metal content.

It can be concluded from this that the higher the basicity (the alkalimetal content) of the glasses, the more effective SO₃ becomes as arefining agent, on account of the SO₂ solubility.

Basic glasses have poor chemical resistances, in particular poorhydrolytic stabilities and poor acid resistances, since their highalkali metal contents can easily be dissolved out of the glass. Forexample, the hydrolytic stabilities of soda-lime glasses only belong tohydrolytic classes ≧3 (DIN ISO 719), and their acid resistances onlybelong to acid classes >2 (DIN 12116).

Sulfate-refined glasses for PDP substrates are also already known. Theseare silicate glasses with a high alkali metal and alkaline-earth metalcontent with little or no boric acid and a high Al₂O₃ content which havehigh thermal expansions. The glasses, with melting points of <1600° C.,are also relatively low-melting and are basic in character.

The patent literature has also already disclosed boron-containingglasses from a wide composition range which, however, according to theexamples have a low SiO₂ content and may also contain sulfate, but onlyin addition to other refining agents. For example, JP 10-25132 Adescribes glasses to which chloride, given as up to 2% by weight of Cl₂,is always added in addition to SO₃, while JP 10-324526 A mentionsglasses to which one component selected from the group consisting ofFe₂O₃, Sb₂O₃, SnO₂, SO₃ and one component selected from the groupconsisting of Cl, F are added in order to reduce the As₂O₃ content.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a process forproducing borosilicate glasses of high hydrolytic stability in which theglass melt is effectively refined, i.e. which results in glass with ahigh quality in terms of absence of bubbles, and which allowsinexpensive, nontoxic refining of the glass melts, in particular glasseswhich melt at high temperatures.

In the process for producing borosilicate glasses belonging tohydrolytic class 1, comprising the process steps of batch preparation,melting of the glass and subsequent hot-shaping, in which context theterm melting, in addition to the actual melting-down of the rawmaterials and cullet, also encompasses the subsequent steps of refiningand homogenization, at least one refining agent, specifically between0.01% by weight and 0.8% by weight of sulfate(s), given in the form ofSO₃, is added to the batch. By way of example, 0.05% by weight of SO₃corresponds to 0.15% by weight of BaSO₄. 0.6% by weight of SO₃corresponds to 2.0% by weight of BaSO₄. It is preferable to addsulfate(s) in an amount of 0.05% by weight to 0.06% by weight of SO₃.

The addition of sulfate(s) initiates the formation and growth of gasbubbles in the glass melt. Even the small quantity mentioned as thelower limit results in effective refining of the borosilicate glasseswith the high hydrolytic stability mentioned (hydrolytic class 1). Thehigh hydrolytic stability is associated with a low basicity, i.e. thesehighly stable glasses have an acidic character.

On the basis of previous knowledge of sulfate refining, it was notpredictable and was indeed altogether surprising that the refiningeffect is sufficiently good in the acidic, relatively high-meltingborosilicate glass melts. This is all the more surprising since thesolubility of SO₂ in acidic borosilicate glasses is very low. Forexample, the SO₃ content in borosilicate glasses is at most approx.0.01% by weight, whereas in soda-lime glasses it is up to 0.5% byweight. The refining action occurs even without the addition of reducingagent. It is even possible for nitrates to be used as raw materials andfor polyvalent compounds to be added in their oxidized form, e.g. Fe₂O₃,without the glass quality being adversely affected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sulfate may be added in the form of one or more sulfates, e.g.MgSO₄, CaSO₄, BaSO₄, ZnSO₄, Na₂SO₄ or other alkali metal and/oralkaline-earth metal sulfate(s); the use of NaSO₄, alone or with BaSO₄,is preferred. The sulfate used must be selected in such a way that therelease of SO₂ and O₂ is matched to the viscosity of the glass meltand/or to the refining temperature of the glass. This is because as yetundecomposed sulfate must be available at the time of refining, and thissulfate then decomposes—without additional reducing agents—to form SO₂and O₂, thereby degassing the glass. If it is released too early, therefining is not sufficient and seeds remain in the glass. The personskilled in the art will readily be able to suitably adapt the relevanttank furnace and melting parameters.

A high acid resistance is also associated with a low basicity.

Therefore, by analogy to the application of the process to glasses witha high hydrolytic stability, it is also surprising and advantageous thatthe process can also be applied to borosilicate glasses with a high acidresistance, i.e. belonging to acid class 1 or 2. The process alsoreveals its very good refining action for glasses of this type and forglasses which belong both to hydrolytic class 1 and to acid class 1 or2.

In the process according to the invention, it is also possible forfluoride and chloride, which serve as fluxes and evaporation refiningagents, to be added as well as the sulfate which is essential to theinvention. For example, the glasses produced using the process maycontain up to 0.5% by weight of F, preferably at least 0.0025% byweight, for preference between 0.005 and 0.4% by weight, of F. Onaccount of the high volatility of the fluorides, these contents mean anaddition of 0.005-1.0, preferably 0.01-0.6% by weight of fluoride, forexample as CaF₂, to the batch.

The glasses may also contain up to 0.3% by weight of Cl⁻. On account ofthe volatile nature of chlorides, this represents an addition of up to0.6% by weight of Cl⁻, for example as NaCl, to the batch. Higher levelswould cause vapors to be released during further processing of theglasses, and these vapors would precipitate as an interfering coating onthe surface (a phenomenon known as lamp rings). It is preferably for atleast 0.015% by weight to be added to glasses. The glasses preferablycontain up to 0.08% weight of Cl⁻. Even the glasses to which no chlorideis added may contain up to 100 ppm of Cl⁻ as an impurity when standardraw materials are used. If particularly pure raw materials are used, itwould be possible to reduce this level to <100 ppm.

The glass produced using the process according to the invention may alsocontain the following polyvalent compounds: up to 5% by weight of Fe₂O₃,preferably up to 2% by weight of Fe₂O₃, up to 1% by weight of CeO₂, upto 5% by weight of MnO₂ and up to 5% by weight of TiO₂.

The process according to the invention is used to produce borosilicateglasses. This term is to be understood as meaning silicate glassescontaining at least 5% by weight of B₂O₃.

The process is used in particular to produce relatively high-meltingglasses which contain at least 65% by weight, preferably at least 70% byweight, particularly preferably more than 70% by weight of SiO₂.

The process is used for the production of borosilicate glasses of a highhydrolytic stability and preferably also a high acid resistance,specifically belonging to hydrolytic class 1 (DIN ISO 719) andpreferably to acid class 1 or 2 (DIN 12116).

Therefore, the process is preferably used to melt glasses selected fromthe following composition range (in % by weight, based on oxide):

SiO₂65-82, Al₂O₃2-8, B₂O₃5-13, MgO+CaO+SrO+BaO+ZnO 0-7, ZrO₂0-2,Li₂O+Na₂O+K₂O 3-10.

The glasses may in each contain the above-mentioned levels of F⁻ and/orCl⁻ as a result of the addition of F⁻ and/or Cl⁻.

The process is preferably used to produce glasses selected from thecomposition range (in % by weight, based on oxide):

SiO₂70-75, Al₂O₃ 4.5-7, B₂O₃9.5-<11.5, MgO 0-2, CaO 0.5-2, SrO 0-3, BaO0-1, ZnO 0-2, MgO+CaO+SrO+BaO+ZnO 1-7, ZrO₂ 0-1, Li₂O 0-1, Na₂O 5-8, K₂O0-3, with Li₂O+Na₂O+K₂O 5-9

The process is particularly preferably used to produce glasses selectedfrom the following composition range (in % by weight, based on oxide):

SiO₂ 72-75, Al₂O₃ 4.5-6.5, B₂O₃ 9.5-<11, CaO 0.5-2, BaO 0-1, Li₂O 0-1,Na₂O 6-8, K₂O 0-<1.5 with Li₂O+Na₂O+K₂O 5-8.

The glasses are preferably produced with the addition of F⁻.

The process is used in particular to produce glasses selected from thecomposition range (in % by weight, based on oxide):

SiO 75-82, Al₂O₃ 2-6, B₂O₃ 10-13, Na₂O 3-5, K₂O 0-1.

The glasses are preferably produced with the addition of Cl⁻.

The process is used in particular to produce glasses belonging to thecomposition range (in % by weight, based on oxide):

SiO₂ 70-75, B₂O₃ 7-10, Al₂O₃ 3-7, Li₂O 0-1, Na₂O 6-8, K₂O 0-3,Li₂O+Na₂O+K₂O 6-10, MgO 0-1 CaO 0-2, BaO 0-4

The process is used in particular to produce glasses belonging to thecomposition range (in % by weight, based on oxide)

SiO₂ 70-76, B₂O₃ 5-13, Al₂O₃ 2-7, MgO 0-1, CaO 0-3, BaO 0-4, ZnO 0-2,MgO+CaO+BaO+ZnO 0-7, ZrO₂ 0-2 Li₂O 0-1, Na₂O 1-8, K₂O 0-6, Li₂O+Na₂O+K₂O4-10

The glasses are preferably produced with the addition of Cl⁻ and F⁻.

The process is used in particular to produce glasses belonging to thecomposition range (in % by weight, based on oxide):

SiO₂ 72-75, Al₂O₃ 5-6, B₂O₃ 7-10, Li₂O 0-1, MgO 0-1, CaO 0.3-1, BaO0-2.5, ZnO 0-3, MgO+CaO+BaO+ZnO 0.3-5, Li₂O 0-1, Na₂O 5.5-7.5, K₂O0-<1.5

The process according to the invention for producing borosilicateglasses of high chemical resistance is therefore preferably used toproduce neutral glasses, i.e. glasses belonging to hydrolytic class 1,glasses for primary pharmaceutical packaging materials, e.g. ampoules,vials, syringes, for producing laboratory glass, laboratory apparatusglass for chemical engineering equipment and pipelines, glasses for lampbulbs, for bioreactors, for biomedical applications, for example forsubstrate glasses for cell culture tests, for producing special glassesin the form of flat glass, cubes, rods, vessels, fibers, granules,powders, for applications in chemistry, laboratory technology,electrical engineering, electronics, e.g. as a sealing glass, and indomestic engineering.

The abovementioned hot-shaping process step encompasses a very widerange of standard hot-shaping methods, such as drawing into tubes orribbons, or floating or rolling, casting, blowing, pressing, as they areapplied according to the intended use of the glass produced, flat orhollow glasses. Here too, the person skilled in the art is readily ableto select a suitable glass composition according to the particularspecification and to select the parameters for the hot-shaping processstep accordingly.

The step of the production process according to the invention which isessential to the invention, i.e. the addition of the abovementionedquantity of sulfate, results in a very effective refining, whichmanifests itself in the excellent glass quality, i.e. the lack ofbubbles and seeds, in the glasses produced, which is also revealed bythe fact that little S can be detected using standard analytical methodsin the finished glasses, i.e. the SO₃ content is <0.01%, meaning thatthe sulfate has been completely or virtually completely converted intoSO₂+O₂ and has left the glass melt in the form of bubbles. This hasbrought about very effective degassing of the glass.

Therefore, the process according to the invention includes effective andin particular inexpensive refining in particular of the glass meltswhich have a high viscosity at the standard refining temperatures andare therefore difficult to refine yet can now be refined to form glasseswith a high glass quality while retaining high melting capacities.

The sulfate-refined products are environmentally compatible on accountof the use of the nontoxic refining agent and their ability to belandfilled is not restricted.

A further particular advantage is that the process according to theinvention does not use large quantities of chloride as refining agent.This makes it possible to avoid the subsequent depositions which arecaused by chloride during further processing and occur as what are knownas lamp rings for example in the case of the glasses for pharmaceuticalapplications which have previously been refined using chloride.

It is preferable to dispense with chloride as a refining agent and forthe glasses produced using the process according to the invention to bechloride-free apart from inevitable impurities.

The process according to the invention for producing borosilicateglasses, unlike the production of soda-lime glasses with sulfaterefining, can be carried out without the addition of reducing agents andmakes do with relatively small quantities of added sulfate.

The invention is to be explained in more detail on the basis ofexemplary embodiments.

As a Comparative Example, a glass having the basic composition (in % byweight, based on oxide) of SiO₂ 74.0; B₂O₃ 10.6; Al₂O₃ 5.7; Na₂O 8.0;CaO 1.3 was melted and refined in a melting end at 1620° C. with theaddition of 0.8% by weight of chloride as NaCl.

The batch was fed continuously, by means of a charging machine, to amelting end, with the quantity supplied being controlled by means of thelevel of the liquid glass in the melting tank. Rough melting, refiningand cooling down of the molten glass were carried out in the usual way.In a working end or a distributor and a subsequent forehearth, the glasswas thermally homogenized and chemically homogenized by stirring.

These individual steps are collectively known by the term melting in thecontext of the description of the invention. The glass was fed via aforehearth to a Danner blowpipe and drawn as a tube. Although the glasscontains few bubbles, disruptive white coatings, known as lamp rings,are formed as a result of the escape of vapors during the furtherprocessing to form ampoules and vials. The number of bubbles cannot bereduced even by lowering the melting capacity by 20%.

As Exemplary Embodiment 1, a glass of the same basic composition as theComparative Example was produced, but 0.46% by weight of Na₂SO₄,corresponding to 0.26% by weight of SO₃, was added instead of 0.8% byweight of Cl⁻. Otherwise, the same raw materials were used and meltingwas carried out with the same melting capacity. The result was agood-quality glass with a similar low level of bubbles to theComparative Example, but unlike in the Comparative Example, this glassdid not present any disruptive lamp rings during further processing.

In a further melt with sulfate refining, it was possible to increase themelting capacity and the throughput by approx. 10% compared to theComparative Example and Exemplary Embodiment 1 without a deteriorationin the bubble quality.

The glass produced using the process according to the invention isenvironmentally friendly, since it does not need toxic refining agents.The process produces glasses of very good quality which do not have anywhite deposits even after further processing. The glasses produced inaccordance with the invention with sulfate refining achieve the samerefining action as with sodium chloride refining. Higher meltingcapacities and higher throughputs are possible.

1. A process of producing a refined borosilicate glass having ahydrolytic class 1, said process comprising the steps of: a) preparing aglass batch with a composition, in wt. % on the basis of oxide content,consisting of: SiO₂ 65-82 Al₂O₃ 2-8 B₂O₃  5-13 MgO + CaO + SrO + BaO +ZnO 0-7 ZrO₂ 0-2 Li₂O + Na₂O + K₂O  3-10

b) adding a refining agent to said glass batch, said refining agentconsisting of from 0.01 wt. % to 0.8 wt. % of at least one sulfate,expressed as an equivalent amount of SO₃, and from 0.005 wt. % to 1.0wt. % of fluoride; c) melting said glass batch including the refiningagent added in step b) to form melted glass; and then d) hot-shapingsaid glass.
 2. The process as defined in claim 1, in which from 0.05 wt.% to 0.6 wt. % of said at least one sulfate, expressed as an equivalentamount of SO_(3,) is added to said glass batch as said refining agent.3. The process as defined in claim 1, in which said refining agentconsists of 0.01 wt. % to 0.8 wt. % of said at least one sulfate,expressed as an equivalent amount of SO_(3,) and from 0.01 wt. % to 0.6wt. % of said fluoride.
 4. The process as defined in claim 1, in whichsaid composition of said glass batch, in wt. % on the basis of oxidecontent, consists of: SiO₂ 75-82 Al₂O₃ 2-6 B₂O₃ 10-13 Na₂O 3-5 K₂O  0-1.


5. The process as defined in claim 1, in which said composition of saidglass batch, in wt. % on the basis of oxide content, consists of: SiO₂70-75 B₂O₃  7-10 Al₂O₃ 3-7 Li₂O 0-1 Na₂O 6-8 K₂O 0-3 Li₂O + Na₂O + K₂O 6-10 MgO 0-1 CaO 0-2 BaO  0-4.


6. The process as defined in claim 1, in which said composition of saidglass batch, in wt. % on the basis of oxide content, consists of: SiO₂70-76 Al₂O₃ 2-7 B₂O₃  5-13 MgO 0-1 CaO 0-3 BaO 0-4 ZnO 0-2 MgO + CaO +BaO + ZnO 0-7 ZrO₂ 0-2 Li₂O 0-1 Na₂O 1-8 K₂O 0-6 Li₂O + Na₂O + K₂O  4-10.


7. The process as defined in claim 1, in which said composition of saidglass batch, in wt. % on the basis of oxide content, consists of: SiO₂72-75 Al₂O₃ 5-6 B₂O₃  7-10 MgO 0-1 CaO 0.3-1   BaO   0-2.5 ZnO 0-3 MgO +CaO + BaO + ZnO 0.3-5   Li₂O 0-1 Na₂O 5.5-7.5 K₂O     0-<1.5 Li₂O +Na₂O + K₂O  5.5-7.5.


8. The process as defined in claim 1, in which said at least one sulfateis one or more components selected from the group consisting ofCaSO_(4,) ZnSO_(4,) MgSO_(4,) Na₂SO₄ and BaSO₄.
 9. The process asdefined in claim 1, in which said at least one sulfate is Na₂SO₄. 10.The process as defined in claim 1, wherein said at least one sulfate isselected from the group consisting of alkali metal sulfates and alkalineearth metal sulfates.
 11. A process of producing a refined borosilicateglass having a hydrolytic class 1, said process comprising the steps of:a) preparing a glass batch with a compositions, in wt. % on the basis ofoxide content, consisting of: SiO₂ 70-75 Al₂O₃ 4.5-7   B₂O₃    9.5-<11.5MgO 0-2 CaO 0.5-2   SrO 0-3 BaO 0-1 ZnO 0-2 MgO + CaO + SrO + BaO + ZnO1-7 ZrO₂ 0-1 Li₂O 0-1 Na₂O 5-8 K₂O 0-3 with Li₂O + Na₂O + K₂O 5-9

b) adding a refining agent to said glass batch, said refining agentconsisting of from 0.01 wt. % to 0.8 wt. % of at least one sulfate,expressed as an equivalent amount of SO₃, and from 0.01 wt. % to 0.6 wt.% of fluoride; c) melting said glass batch including the refining agentadded in step b) to form melted glass; and then d) hot-shaping saidglass.
 12. The process as defined in claim 11, in which said compositionof said glass batch, in wt. % on the basis of oxide content, consistsof: SiO₂ 72-75 Al₂O₃ 4.5-6.5 B₂O₃  9.5-<11 CaO 0.5-2   BaO 0-1 Li₂O 0-1Na₂O 6-8 K₂O     0-<1.5 with Li₂O + Na₂O + K₂O  5-8.


13. The process as defined in claim 11, wherein said at least onesulfate is selected from the group consisting of alkali metal sulfatesand alkaline earth metal sulfates.
 14. A process of producing a refinedborosilicate glass having a hydrolytic class 1, said process comprisingthe steps of: a) preparing a glass batch with a composition, in wt. % onthe basis of oxide content, consisting of: SiO₂ 72-75 Al₂O₃ 4.5-6.5 B₂O₃   9.5-<11.5 CaO 0.5-2   BaO 0-1 Li₂O 0-1 Na₂O 6-8 K₂O     0-<1.5 withLi₂O + Na₂O + K₂O  5-8;

b) adding a refining agent to said glass batch, said refining agentconsisting of from 0.01 wt. % to 0.8 wt. % of at least one sulfate,expressed as an equivalent amount of SO₃, and from 0.01 wt. % to 0.6 wt.% of fluoride; c) melting said glass batch including the refining agentadded in step b) to form melted glass; and then d) hot-shaping saidglass; wherein said at least one sulfate is selected from the groupconsisting of alkali metal sulfates and alkaline earth metal sulfates.